Prior art infrared (IR) imaging systems typically include a planar detector area, which is also known as the Focal Plane Array (FPA), which is planar detector pixels on a planar substrate that typically includes a Read Out Integrated Circuit (ROIC). The planar detector area further usually consists of a plurality of pixels that are mostly thermally isolated from, but electrically connected to, the substrate via mechanical isolation legs. The pixels act as microbolometers, in that IR energy from the scene changes the pixel temperature, which further changes the pixel resistance. For each pixel, the change in resistance across the isolation legs is detected, measured, and represented by support circuitry, both in the substrate and other support circuit boards, to generate an IR image.
For most IR imaging systems, there are several characteristics that are extremely desirable. Specifically, it is desired that the systems have the best sensitivity achievable. It is also desirable that the IR images generated have a high resolution for any given Field of View (FOV). It is also generally desirable that IR imaging systems have a larger FOV if task-required resolution can be retained. The shape of the pixels, as well as the arrangement thereof within the FPA, can affect these attributes, in that an FOV increase requires an increase in the number of pixels in the FPA and how closely they are located to each other if resolution and performance are to be retained.
The FPA unit cell Fill Factor is the ratio of active absorption area to unit pixel cell size. Unit cell Fill Factor can influence how the detector pixels are arranged on the FPA and the number of pixels per unit area. Different pixel shapes and different pixel arrangements could increase the unit cell Fill Factor and also fill some of the non-imaging real estate of the FPA with active absorption area. Such arrangements which improve the Fill Factors of the unit cell and the FPA could provide increased resolution for the imaging device, and could improve IR imaging performance in microbolometer based systems using single or multiple layer pixel designs.
For prior art IR devices, the microbolometer pixels usually have a rectangular planar absorption area and are arranged in straight, perpendicular rows and columns. Further, the isolation legs typically extend outwardly from the pixel perimeter for a single layer structure, or are folded under the absorption area in right angle traces for a multiple layer structure. The rectangular shape of the current pixel and isolation leg structure would not greatly benefit from a staggered pixel arrangement in terms of FPA fill percentage. Although a staggered row or column design may help IR imaging system performance with rectangular pixels in terms of image sampling, it would be of further benefit to change the shape of the pixel and allow them to be placed more closely together on the FPA.
In some instances, it is desirable to have an increased Field Of View (FOV) for the IR imaging system, which could result in increased angles of incidence for the incoming IR radiation energy from the scene depending on the system front end optics. Absorption, and therefore sensitivity, could be improved if the incoming radiation is as orthogonal as possible to the pixel surface.
Superimposed over these considerations is the fact that microbolometer pixels are coincident with spaced-apart parallel planes, which establishes a tuned resonant cavity for the wavelength of interest between the FPA and the substrate for the device. During operation, a portion of the incoming IR radiation passes through the pixel absorption surface and reflects off of the substrate to be absorbed again by the pixel. If pixel shapes are changed, then the resonant cavity efficiency of operation could be changed accordingly and perhaps adversely.
One way to increase pixel absorption, especially at increased angles of incident radiation, would be to curve the absorption surface area of each microbolometer pixel in one or both directions. A method of decreasing the distance between pixels on the FPA would be to maximize the unit cell fill factor and pursue alternative shapes to the current rectangles.
In light of the above, it is an object of the present invention to provide an IR imaging device with pixels having round absorption areas, to allow for different pixel arrangement on the substrate to create more densely packed FPAs. It is another object of the present invention to provide an IR imaging device with a higher fill factor per unit area. Another object of the present invention is to provide an IR device with a curved absorption surface to improve the absorption of the device. Yet another object of the present invention is to provide an IR imaging device capable of maintaining sensitivity with an increased Field Of View. Another object of the present invention is to provide an IR imaging device that is relatively easy to manufacture in a cost-effective manner.